POS stands for Point Of Sale or Point Of Service.
It refers to the time and place where the customer delivers payment for the exchange of a good or service.
Point of Service has become a more modern adaptation of Point of Sale as it encompasses the entire service relationship – including repeat customers and orders.
It can also mean the software or hardware [equipment] used at the POS. So in a retail store or restaurant this would include the register which processes the transaction, EFTPOS machine, scales, scanners, and receipts. As well as the front of house elements, POS can refer to inventory management or fulfilment systems, CRM [customer relationship management], and financial records etc.